In order to inspect film labels shrunk on bottles or other containers, it is known e.g. from patent DE 10 2005 023 534 B4 to conduct bottles to be inspected, which are placed on a linear conveyor belt, through an inspection unit comprising two cameras and a mirror cabinet so as to image the whole circumference of the label simultaneously. The apparatus described further comprises an arena-like arrangement of lateral light screens, the illumination-side and the imaging-side beam paths having provided therein linear polarizing filters with intersecting polarizing planes so as to suppress disturbing reflections of the light screens in the recorded images.
When the film labels are inspected with respect to an existence of holes and/or fissures, the problem arises, in particular in areas of the label whose color is similar to that of the material filled in the container, that such faulty locations can only be detected with very great effort and/or cannot be detected reliably because the contrast between such locations and the surrounding label is weak.
Up to now, attempts have been made to solve this problem by comparing the acquired image data with reference pictures of the shrunk-on label. In practice, this is, however, difficult due to the fact that the position of the label on the bottle may vary, a circumstance leading, especially in the case of bottle surfaces with non-uniform curvatures, to different distortions of the label imprint. Hence, large tolerance ranges between the inspected label and the reference have to be admitted, whereby the reliability of recognition will be reduced. In addition, when the labels are changed frequently and/or in the case of changes in the imprint, many reference pictures and additional calibrations are necessary, which are cumbersome to handle.